Layer-dependent exciton formation in two-dimensional crystals of perylene derivatives

Organic crystals have garnered attention due to their flexibility, high strength, and their synthesis over large areas using diverse materials. Perylene derivatives, such as 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA), are typical organic semiconductors with a layered structure and high carrier mobility. Although perylene derivatives' exciton formation process has been extensively studied^{1, 2}, the mixing and conversion between excitons is only roughly known. Here, we observed changes in exciton formation in PTCDA as a function of layer number using time-resolved spectroscopy. We fabricated several layers of PTCDA through physical vapor deposition and determined their exciton lifetimes using time-resolved fluorescence. Precisely controlled 2-layer PTCDA enables us to directly observe emission of charge-transfer exciton (CTE). Additionally, the appearance of excimers in thicker samples, along with a significant decrease in the fluorescence lifetime of CTE, confirms the conversion of CTEs to excimers under specific conditions. This study not only showed the formation of excimers but also quantitatively identified the conditions for their formation. This could facilitate a better understanding of the exciton properties of organic crystals, which may have implications for various optoelectronic device applications, including OLEDs.